The purpose of the program of research outlined in this grant proposal is to determine whether human infants in the second six months of life are able to perceptually segment fluent speech into word-length units that could be used to understand their native language. Although many studies in the past 15 years have documented the sophisticated speech discrimination capacities of young infants, those studies have focussed on the infant's perception of isolated speech segments. Studies of speech segmentation based on speech production are limited by the poor articulatory control shown by infants once words are produced and by the fact that perceptual segmentation must precede word production. In contrast to the perception of isolated speech segments, the perception of fluent speech requires on-line processing ant the extraction of discrete acoustic units from a speech waveform characterized by inconsistent acoustic markers for word boundaries. Two questions are critical: (a) can infants extract a familiar acoustic unit from fluent speech and recognize the equivalence of this unit in various contexts, and (b) is this extraction process facilitated by the entry of acoustic units into the lexicon? Three strategies will be employed to study speech segmentation in 6- to 12-month-olds. First, an operant headturning procedure and a visual habituation procedure will be used to assess infants' extraction of word-length units from fluent speech. Variables though to influence segmentation (e.g., speaking rate, intonation and stress, coarticulation of adjacent phonetic segments) will be examined on these segment extraction tasks. Second, the speech INPUT to the infant from the mother will be analyzed in great detail to describe the variability and consistency with which acoustic cues to segmentation are presented to the infant. Short-term "training" studies will assess the role of reference in infants' segmentation of words and pseudowords. Third, infants' preferences for variations in speaking rate and stress patterns will be obtained to determine whether these suprasegmental aspects of fluent speech influence infants' attention. Taken together, these studies will clarify an under-studied but essential aspect of language required for lexical and syntactic development.